The present invention relates generally to minimizing impedance in capacitors and more particularly to low resistance, low inductance capacitors having low manufacturing costs and high capacitance to volume ratios.
All capacitors have series equivalent circuits with impedance consisting of capacitance, resistance, and inductance parameters, and conventional electrolytic capacitors have resonant frequencies between 5 and 100 kilohertz depending on size, voltage rating, and construction. At frequencies below the resonant frequency, the capacitor impedance is primarily affected by the capacitance and is minimized by maximizing capacitance. While the total resistance effect, or equivalent series resistance (ESR), has minimal influence on the impedance except at frequencies near the resonant frequency, the effect of ESR is noticeable in heat generation at all frequencies. The effect of the ESR is minimized by minimizing the ESR. Above the resonant frequency, the capacitor impedance is primarily affected by the total inductance, or equivalent series inductance (ESL) and is minimized by minimizng the ESL.
In the past, the capacitors used in conventional D.C. power supplies, which rectified 60 hertz line voltage to supply the desired D.C. level operated in ranges below the resonant frequency and the effects of the ESR and ESL were minimal. With the increased acceptance of switching regulated D.C. power supplies which operate at frequencies between 10 and 100 kilohertz (with resulting 20 to 200 kilohertz ripple and harmonics), new low ESR and ESL capacitors are required because normal operation is in ranges at or above the resonant frequencies.
Various other capacitors have been developed having relatively low ESR's and low ESL's such as those shown in the Puppolo, et al, U.S. Pat. No. 3,822,397 and the Voyles, et al, U.S. Pat. No. 3,806,770. However, since the terminals have not been side by side, or juxtapositioned, along their entire lengths inside the case, significant impedance still occurs in these type capacitors. Further, the split terminal cover design of the Puppolo patent and the tantalum capacitor design of the Voyles patent are expensive to manufacture and have low capacitance to volume ratios.
Also, in switching regulated power supplies with previous capacitors, additional capacitors having higher ESL's were required to filter out the 20 to 200 kilohertz ripple and harmonics fed to the load and from the switching regulator back into the main 60 hertz power supply line.